The present invention relates to a thermal type flow meter, and more particularly, the invention relates to a thermal type flow meter which is suitable to detect a quantity of intake air in an engine for automobiles, and to a measurement element for the flow meter.
In an engine for automobiles, it is required to operate with stability at temperatures within extremely wide range. Generally, the range of temperatures is from -30.degree. C. to +80.degree. C. Accordingly, in a thermal type flow meter for an engine, it is essential to be able to measure air flow with accuracy within the above-mentioned range of temperatures.
However, as is well-known, the physical value of air vary according to the air temperature. Therefore, in the constant temperature type hot wire wind gauge, an error occurs with a variation of the temperature of the intake air. In such a flow meter, because the flow quantity or an output can be converted into a value proportional to the measured flow velocity, the flow velocity or the wind speed also can be obtained as a measure of a flow quantity or a wind quantity.
The variation of the physical properties of air corresponding to a variation of temperatures within the above-mentioned temperature range is shown in FIGS. 10 and 11. FIG. 10 is a numerical value graph showing the variation of various physical properties of air corresponding to a variation of the temperatures. FIG. 11 is a numerical value graph showing the variation of flow velocity, a non-dimensional number and thermal conductivity corresponding to a variation of various physical properties of air which correspond to a variation of temperatures. For example, if the physical properties at 25.degree. C. (designated by a final letter O), which is a center value of the above temperature range, is set as a standard value or reference value, the density .sigma.a of air, the coefficient .nu.a of kinematic viscosity, the heat conductivity .lambda.a and the Prandtl number Pr vary as shown in FIG. 10. As a result, the flow velocity ua, the Reynolds number Re, the average Nusselt's number Num and the average heat-transfer coefficient .alpha.m vary as shown in FIG. 11.
The average heat-transfer coefficient .alpha.m, which has an effect on the final measurement, becomes larger as the air temperature increases, and it becomes smeller as the air temperature decreases. Basically, the variation of the average heat-transfer coefficient causes a variation of the quantity of heat-radiation from the heating resistor and the support member for it, namely, a variation of the total heating quantity of the thermal type flow meter. This causes an error of the thermal type flow meter due to the variation of the temperature of the intake air. In general, this error is compensated by providing an approximately constant heating extent (over-temperature .DELTA.Te against the intake air temperature) by using a bridge circuit, or an electronic circuit having a function equivalent to a bridge circuit, connected to the flow meter, so that the temperature The of the heating resistor or the electrical resistance Rh may vary according to the intake air temperature.
Japanese Patent Laid-Open No. 55-50121 (1980) discloses a technique in which the temperature of a heating resistor is varied according to the detected intake air temperature, in order to solve the above-mentioned problem. Namely, it states that by detecting the intake air temperature, using a temperature compensating resistor, and providing approximately constant heating or the over-temperature .DELTA.Te to the heating resistor for detecting the flow velocity by using a bridge circuit, this error can be eliminated.
Further, Japanese Patent Laid-Open No. 5-312612 (1993) points out that the Japanese Patent Laid-Open No. 55-50121 does not consider compensation of the loss of heat radiation and that of heat-transfer to the support portion of the heating element. In order to compensate mainly the loss of heat radiation, the equipment disclosed in Japanese Patent Laid-Open No. 5-312612 is so constructed that a second measurement element, in addition to first measurement element, is arranged in a fluid passage, and the second measurement element may be heated more than the temperature of the first measurement element in a low flow velocity range when the temperature of the fluid is high.
Further, Japanese Patent Laid-Open No. 4-285818 (1992) discloses a technique in which the flow quantity in a wide range can be accurately detected by setting the value of the temperature coefficient of the electrical resistance of a temperature compensating resistor Rc to a value lower than that of the temperature coefficient of the electrical resistance of a heating resistor Rh.
Japanese Patent Laid-Open No. 5-52626 (1993) discloses a technique in which high responsibility and high adhesive strength of a lead wire can be obtained by joining lead wires, having a core wire consisting of a material having a heat conductivity lower than that of platinum coated with an alloy layer of which the main component is platinum, for example, 40 Ni--Fe alloy or SUS 430, to a heating resistor having a body which is formed by a bobbin type ceramics.
While, in the prior art disclosed in Japanese Patent Laid-Open No. 55-50121, the component of the variation of the heat transfer quantity due to a compulsive convection current is compensated with respect to temperature, no consideration is given to the fact that the variation of the characteristics of the heat transfer of the flow meter, including the heating resistor and its lead, must be compensated. Even if the adjustment for eliminating an error, for example, at an intermediate and arbitrary flow velocity or flow quantity, is performed, the error becomes large in a measurement range apart from the adjusted values (due to the fluctuation of the characteristics of the heat transfer of the heating resistor in accordance with the flow velocity or the flow quantity). This is concretely shown in FIG. 9, which is a numerical value graph showing a variation of error in an output of the flow conversion according to a variation of the flow velocity at different intake air temperatures in the conventional thermal type flow meter. As designated by a solid wire representing the result of measurement in FIG. 9, when the intake air temperature varies from 25.degree. C. to 80.degree. C., there is a problem that a plus error occurs at the low flow side and a minus error occurs at the high flow side.
Further, the technique disclosed in Japanese Patent Laid-Open No. 5-312612, which can solve the problem of the approach taken in Japanese Patent Laid-Open No. 55-50121, has the problem that the hardware becomes complicated. According to the inventors' study, the main cause of a variation of the error due to the flow velocity or the flow quantity is not based on the variation of loss of the heat radiation, but is caused by the variation of loss of the heat transfer to the support member for the heating element shown in the Japanese Patent Laid-Open No. 5-312612. It is, therefore, impossible to sufficiently compensate the output error against a variation of the flow velocity due to a variation of the heat transfer to the support member for the heating element by using the above-mentioned construction. The reason for this will be explained hereinafter.
FIG. 8 shows a temperature compensating circuit in the conventional thermal type flow meter. For example, for the conventional thermal type flow meter, in order to compensate the temperature of the heating resistor by using a bridge circuit as shown in FIG. 8, consideration is given to the variation of the temperature The of the heating resistor, the temperature Thl of the lead and the temperature Tht of the terminal at the standard temperature of 25.degree. C. of the intake air and at a temperature of 80.degree. C., as shown in FIG. 12. Further, the variation of the ratio of the total heating quantity Qt (80) at an intake air temperature of 80.degree. C., the ratio of the radiation quantity Qa (80) to the air in an element body and the ratio of the heat transfer quantity Q1 (80) to the lead to the total heating quantity of the flow meter at a standard intake air temperature of 25.degree. C., which occur due to the variation of the flow velocity, is shown in FIG. 13.
As shown in FIG. 12, while the temperature The of the heating resistor is approximately constant at 25.degree. C. and 80.degree. C. with respect to the flow velocity, the temperature Thl of the lead and the temperature Tht of the terminal becomes high as the flow velocity becomes low. Further, it is seen from FIG. 13 that Qa(80)/Qt(25) increases as the flow velocity increases, that Q1(80)/Qt(25) decreases as the flow velocity increases. The sum Qt(80)/Qt(25) is relatively large at the side of the low flow velocity. This small inclination finally causes a plus error at a low flow and a minus error at a high flow. This is because the inclination of the variation Q1(80)/Qt(25) of the heat transfer to the lead wire is larger by a little than the inclination of the variation Qa(80)/Qt(25) of the radiation quantity from the element body.
This fact is clearly seen from FIG. 14, which is a numerical value graph showing a variation of the difference between the ratio of the heating quantity at different intake air temperatures and a variation of the temperature of the heating element in the conventional thermal type flow meter. FIG. 14 shows the variation of the difference .delta..delta. between the ratio Q1(80)/Qt(25) of the heat transfer quantity Q1(80) to the lead at an intake air temperature of 80.degree. C. and the total heating quantity Qt(25) at an intake air temperature of 25.degree. C at a flow velocity of 50 m/s, and the ratio Q1(80)/Qt(25) at a low velocity, caused by the variation of the flow velocity. The solid wire designates an example of the conventional thermal type flow meter, in which the value .delta..delta. at a flow velocity of 0.5 m/s is more than 50% of the value at a flow velocity of 50 m/s.
The operation of the apparatus disclosed in the Japanese Patent Laid-Open No. 4-28581 is basically the same as that of the Japanese Patent Laid-Open No. 55-50121. Therefore, there is the problem that the variation of the inclination of the error which occurs due to the variation of the flow velocity under different conditions of temperature is not eliminated, even if temperature compensation is performed at a certain flow. In this reference, as means for varying the coefficient of temperature of the electrical resistance by using the temperature compensating resistor and the heating resistor, there is provided means for changing the thickness of a metal film or the condition of heat treatment. However, such a means is not practical when taking the variations which occur in production into consideration.
Further, in the technique disclosed in the Japanese Patent Laid-Open No. 5-52626, the heat transfer from the heating resistor to the lead member is decreased by lowering the thermal conductivity to the lead member. Namely, the improvement of the variation of the thermal conductivity at the lead which causes the variation of the measurement error variation due to the variation of the flow velocity in the flow meter is not considered.
Recently, there has been an increased need to suppress the direct or indirect effect of the harmful components included in exhaust gas emitted from an automobile, to reduce their effect on the environment. In the United States, it is a policy and a requirement in some jurisdictions that the maximum reference value of the emission amount of NOx, out of the harmful components CO, HC and NOx included in the exhaust gas emitted from an engine for an automobile, must be decreased to one half at present, and then to one third by the year 2000. Similarly, taking the future of the world petroleum supply into consideration, it is a policy that the maximum reference value of the percentage of fuel consumption must be decreased to one half after the year 2000. The above-mentioned harmful components and the fuel consumption is extremely effected by the air/fuel ratio, which is the mixture of the air and the fuel burned in an automobile engine. Accordingly, it is necessary to control air/fuel ratio more accurately than ever in the operation of an automobile engine, and this makes it necessary to improve the accuracy of the flow measurement which directly determines the adjustment of the air/fuel ratio. Particularly, in conjunction with the use of catalyst to decrease the above-mentioned harmful components, the air/fuel ratio is controlled in the neighborhood of the value 1, because the invert ratio sharply varies from approximately 100% after and before the value of the air/fuel ratio. Therefore, in order to satisfy the above-mentioned reference, it is necessary to decrease the current measurement error to one half or one third.